A lithium ion battery, of which a lithium ion secondary battery is representative, is widely prevalent as a main electric power source of a mobile electronic apparatus (for example, a smartphone, a mobile telephone or a notebook PC). For a lithium ion battery like this, in order to achieve further high density, high capacity and high output of energy, technological development is underway.
As the negative electrode material of the lithium ion battery, a carbon material, a compound or the like which is able to store and release lithium ions is utilized. As the positive electrode material of the lithium ion battery, LiCoO2 or the like is utilized. As the electrolyte liquid of the lithium ion battery, one such that LiPF6 or the like as an electrolyte is dissolved in an organic solvent such as ethylene carbonate or the like is utilized. As the separator of the lithium ion battery, polyethylene (PE) or the like is utilized.
The separator is a member which physically separates the positive electrode and the negative electrode. In a case where the separator is of a fiber type, the diameter of the fiber, which is a configuration member, largely affects movement or retention of the electrolyte liquid. The void rate of the separator becomes small if the diameter of the fiber is large and, since the liquid amount of the electrolyte liquid which is able to be retained becomes little, the ion mobility between the positive and negative electrodes at the time of electric charge/discharge becomes bad. Because of that, development of separators utilizing nanofiber fibers with a diameter of the fiber being narrow has been recently required.
Moreover, for a lithium ion battery which is configured from these materials, in a case where an abnormal electric current flows by a short-circuit and the like, the battery temperature remarkably rises, and there is a possibility that a thermal damage could be given to the apparatus.
Accordingly, in a case where a rise of the temperature by an abnormal electric current occurs, by allowing the electric resistance of the separator to increase, the battery reaction is blocked, and a remarkable rise of the battery temperature is prevented.
In general, called a shutdown characteristic is the function of ensuring safetiness by blocking, in this way, the battery reaction by the increase of the electric resistance on the occasion of a temperature rise of the lithium ion battery so that a remarkable rise of the battery temperature is prevented. This shutdown characteristic is an important characteristic for the separator of the lithium ion battery.
Generally, for the shutdown function of the separator, the operating principle is that microporous-membrane-shaped polyethylene melts and stops up porous of the separator. Because of that, for example, in a case of a separator which is consisting of polyethylene, the shutdown function operates approximately at 140° C. in the vicinity of the melting point of polyethylene.
Here, proposed is a separator of a lamination type such that, for the separator with the shutdown function working, which contracts by a remarkable rise of the battery temperature, the positive electrode and the negative electrode are not short-circuited (for example, see Japanese Patent Application Publication No. 2010-103050). For the separator of Japanese Patent Application Publication No. 2010-103050 being for a lithium ion battery, integrated with lamination are a base material, a nanofiber layer which is formed on this base material and includes heat-resistance-property polymer, and a low-melting-point nanofiber layer which is formed on this heat-resistance-property nanofiber layer.
FIG. 10 is for the separator of Japanese Patent Application Publication No. 2010-103050. As shown in FIG. 10, for the separator of Japanese Patent Application Publication No. 2010-103050, formed on the base material 30 is the heat-resistance-property nanofiber layer 32 the melting point of which is 180° C. or more, and formed on the heat-resistance-property nanofiber layer 32 is the low-melting-point nanofiber layer 31 the melting point of which is 80-120° C. The separator of Japanese Patent Application Publication No. 2010-103050 is a separator being for a lithium ion battery such that these laminated layers are fused with a heat press.